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96

Information Notice No. 93­96: Improper Reset Causes Emergency

Diesel Generator Failures

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

December 14, 1993

NRC INFORMATION NOTICE 93‐96: IMPROPER RESET CAUSES EMERGENCY DIESEL

GENERATOR FAILURES

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to the potential for an emergency diesel generator

(EDG) to fail to start on demand or to trip during operation due to improper

execution of the reset operation at the conclusion of the previous run. It is

expected that recipients will review the information for applicability to

their facilities and consider actions as appropriate to avoid similar

problems. However, suggestions contained in this information notice are not

NRC requirements; therefore, no specific action or written response is

required.

Description of Circumstances

H.B. Robinson

In August of 1987 at the H.B. Robinson plant, startup trips occurred on the

"A" EDG. With the assistance of a vendor representative, several steps were

taken in an effort to preclude such trips, including replacement of the

governor, replacement of the governor oil, replacement of sticky fuel

injection pumps, checking of injection pump timing, and rebalancing fuel rack

settings. In February of 1988, startup trips were again experienced. It was

concluded, after detailed inspection of the latch mechanism parts, that there

was a possibility of wear on the tip of the latch, which may have been a

contributing factor to the trips. These parts were replaced with new parts.

On June 3, 1992, during the performance of a test activity, the "A" EDG

tripped on mechanical overspeed after the engine had been operating

approximately one half hour. After detailed consideration of possible

scenarios for the trips and considering prior actions taken, it was determined

that the most probable cause was a failure to properly reset the trip

mechanism after the previous run. It was concluded that operations personnel

apparently were unaware of the importance of moving the reset lever through
its full travel. In addition, resetting was difficult for shorter individuals

due to the location of the reset lever, which is positioned over the exhaust

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IN 93‐96

December 14, 1993

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manifold. Operations personnel were subsequently trained on the proper method

for actuating the reset lever. Also, a step was provided to permit easier

access to the lever.

Duane Arnold

On June 11, 1993, while the Duane Arnold plant was operating at 85 percent of

full power, the "A" EDG tripped approximately 3 seconds after the initial

start signal during an operability surveillance test. An evaluation was

conducted by the licensee of possible EDG trip causes, including occurrence of

an actual overspeed condition, failure of the overspeed mechanism, actuation

of the emergency stop pushbutton, failure of the emergency stop pushbutton,

latch assembly not fully latched, latch assembly failure, mechanical trip

mechanism failure, and annunciator circuit or trip lever microswitch failure.

It was concluded that the most probable cause of the trip was that the latch

assembly was not fully engaged from the previous reset operation and engine

startup vibration unlatched the trip mechanism. Less than complete engagement

of the latch assembly could be caused by an inadequate reset operation.

Corrective actions adopted by the licensee included procedure revisions to

ensure a more positive latching operation, improved physical access to the

reset lever, and operator training.

The procedure revision directs operators to perform the reset operation a

second time following the initial resetting of the trip. The second reset is

somewhat easier to perform and should ensure that the latch is fully engaged

in the event that the initial latching sequence resulted in a partially

latched condition. Verification that the latch is fully seated is

accomplished by checking that the reset lever subsequently moves freely.

Discussion

In response to experience with previous trips during fast starts, the diesel

manufacturer for both plants, (Coltec/Fairbanks Morse Engine Division), had

previously issued a service information letter (SIL, Volume A, Issue 25, dated

August 1, 1991) addressing a revised overspeed trip adjustment. The SIL

recommended that the overspeed trip setting be raised from the range of 112 to

115 percent to the range of 115 to 117 percent. In light of events that

occurred at H.B. Robinson and Duane Arnold, the manufacturer has now advised

these licensees that proper resetting of the trip mechanism would be enhanced

by slowly moving the reset lever fully to the reset position and back, twice.

The inadvertent partial reset latching operations described above may also

occur on other units having similar mechanical trip mechanisms.

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IN 93‐96

December 14, 1993

Page 3 of 3

This notice requires no specific action or written response. If you have any

questions about the information in this notice, please contact one of the

technical contacts listed below or the appropriate Office of Nuclear Reactor

Regulation (NRR) project manager.
/s/'d by BKGrimes

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contacts: Larry Garner, RII

(404) 331‐4663

Chris G. Miller, RIII

(319) 851‐5111

Walter P. Haass, NRR

(301) 504‐3219

Attachment:

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